1. Field of the Invention
This invention relates in general to filtering hydraulic fluid in hydraulic systems, and in particular to a filter for the hydraulic fluid supplied to a surface or subsurface control valve on an oil or gas well.
2. Description of the Prior Art
High pressure oil and gas wells employ hydraulically actuated safety valves for the control of the well. The valve is located in the tubing below the surface. Hydraulic fluid pressure is supplied to the valve to keep the valve open. In the event that hydraulic fluid pressure fails, the valve automatically closes to prevent further flow of production fluid up the tubing.
Particularly for high pressure applications, 15,000 psi to 20,000 psi, it is important to keep the sealing areas of the valve hydraulically actuated components free of any debris. Otherwise, leakage past the valve might occur. It is also important that the valve be free to close very quickly once the hydraulic fluid pressure is relieved.
Filters are employed in prior art hydraulic systems in general to filter hydraulic fluid. However, generally the filters do not have a high density. As a result, smaller particles of debris may flow past the filter. The reason that the filters are not of very high density is to prevent the filter from creating too much of a pressure drop. A high density filter would impede the reverse flow of hydraulic fluid in the event of a hydraulic pressure fluid loss on a well safety valve system. This impediment to the reverse flow of hydraulic fluid would slow the closing of the valve.